COMPOSITION, PROPERTIES, AND TESTS OF PLASTERS. 



65 



house tankage are some of the retarders most commonly used. Many 

 " patent" retarders are also on the market, most of which are based 

 on one or more of the organic materials noted above. 



In the course of the experiments by Slosson and Moudy, recorded 

 below, an effort was made to obtain a cheap and satisfactory home- 

 grown retarder. With this in view a common western cactus (Opuntia 

 platycarpia) was dried and ground. The common malva (Malvastrum 

 coccineum) was also prepared in a similar manner. The retarders thus 

 made were light green in color, possessed no disagreeable odor, and when 

 used in the proportion of 2 pounds of retarder to a ton of plaster gave 

 excellent results. It is probable that many other plants, dried and 

 ground in similar fashion, would form satisfactory retarders. 



Effect of retarders on strength of plasters. The following table * 

 contains the results of compression tests by Profs. Slosson and Moudy 

 on plasters containing various amounts of retarder. It appears to 

 show conclusively that the compressive strength (and inferentially the 

 tensile strength) of plasters decreases as the amount of retarder in- 

 creases. 



TABLE 18. 

 EFFECT OF RETARDERS ON STRENGTH OF PLASTERS. 



In making the tests the finely ground retarder was accurately 

 weighed and thoroughly mixed with the dry plaster. Water was then 

 added and the mixture, placed on a glass plate, was fashioned into a 

 pat about 4 inches in diameter. For determining the setting time 

 two needles were used, each -^ inch in diameter, one loaded with a weight 

 of \ lb., the other with a weight of 4 Ibs. The time when the more 

 lightly weighted needle ceased to make a decided impression on the 

 surface of the plaster pat is reported as "initial set", and when the 

 more heavily weighted needle was supported without indenting the 

 plaster is reported as " final set ". 



* Tenth Ann. Rep. Wyoming Agric. and Mech. College. 



